The present invention relates to a 16-bit microprocessor system having a 16-bit microprocessor and an external segment/page table to provide an address space exceeding 64 KB (kilobytes) by segment/page processing.
16-bit microprocessors have higher efficiency and better functions than 8-bit microprocessors and are more economical than 32-bit microprocessors. Therefore, 16-bit microprocessors have become widespread and now play a main role in microprocessor systems. However, the systems have a disadvantage in that the size of the address area has an upper limit of 2.sup.16 (i.e., 64 KB). This is because 16-bits are the basic unit of data processing, not only between the 16-bit microprocessor and peripheral devices, but also within the 16-bit microprocessor. In order to solve this problem, a segment or page register can be added to expand the 16-bit address space.
In address calculations such as index modification, the effective addresses are calculated as a modulo-N residue (N=2.sup.16). However, even if segment/page processing is applied, the address space of 64 KB cannot be widened unless segment/page switching is performed. In other words, in order to obtain an address space exceeding 64 KB by segment/page processing, segment/page switching must be performed.
However, in data processing, storing a great amount of continuous data in graphic or image processing, the segment and page cannot be sequentially switched in accordance with a program. For this reason, it is very difficult to use a 16-bit microprocessor system in graphic or image processing.
Demand has recently arisen for a 16-bit microprocessor system capable of accessing a continous data area exceeding 64 KB without performing segment/page switching. In this case, it is preferable to use a current OS (operating system) to which segment/page processing is applied. This is due to the importance of effectively utilizing widespread software sources such as UNIX as a 16-bit standard OS. UNIX was developed by Bell laboratory (BTL) and serves as an OS for a PDP 11 available from DEC. UNIX is a trademark of the Bell laboratory.